In recent years, from the viewpoint of the global environmental conservation, the development has been progressed for fuel cells of the type that has high electric power generation efficiency and that do not emitting CO2. Fuel cells of this type generate electricity by causing the reaction between H2 and O2. Depending on the type of electrolyte, the fuel cells under development are classified by the type of electrolyte into, for example, phosphoric acid type fuel cells, molten carbonate fuel cells, solid oxide fuel cells, alkaline type fuel cells, and proton-exchange membrane fuel cells.
Of these cells, compared with other fuel cells, a proton-exchange membrane fuel cell has the following advantages:
(a) The fuel cell has an electric power generation temperature of 80° C.; that is the cell is capable of generating electricity at a significantly low temperature;
(b) The mainbody of the fuel cell can be formed to be light and compact; and
(c) The fuel cell can start up in a short time.
The proton-exchange membrane fuel cell is, therefore, one of today's most attractive fuel cells that are appreciated for use as electric vehicle on-board power sources, home-use stationary power generators, and portable compact power generators.
A proton-exchange membrane fuel cell of this type generates electricity from hydrogen and oxygen through polymer membranes. As shown in FIG. 1, the cell is structured such that a membrane-electrode assembly 1 is clamped between gas diffusion layers 2 and 3 (carbon paper, for example) and further between separators 4 and 5 to form to be a single constitutional element (so-called a “unit cell”), wherein electricity force is generated between the separators 4 and 5.
The membrane-electrode assembly 1 is called “MEA (or, membrane-electrode assembly) that is formed by integration of a polymer membrane and an electrode material such as a carbon black carrying a platinum-based catalyst on obverse and reverse surfaces thereof and have a thickness of from several tens of micrometers (μm) to several hundreds of micrometers (μm). Many alternative cases can be seen where the gas diffusion layers 2 and 3 are integrated with the membrane-electrode assembly 1.
In the case of adaptation of such the proton-exchange membrane fuel cell for the use as described above, the fuel cell is used in the form of a fuel cell stack that is formed by series connecting several tens to several hundreds of unit cells as described above.
The separator 4, 5 is required to have functionalities as
(A) A barrier wall to play the role of separating between unit cells;
(B) A conductor for carrying generated electrons;
(C) An airflow channel or hydrogen flow channel along which O2 (i.e., air) or H2 flows; and
(D) A discharge channel along which generated water, gas and the like flows.
Further, for the separators 4 and 5, separators having high properties such as high durability and electrical conductivity are required to be used for practical using of the proton-exchange membrane fuel cell.
Regarding the durability (i.e., output voltage fall withstanding property), when used as an electric-vehicle on-board power source, the fuel cell is contemplated to have a durability time of about 5000 hours. As an alternative case where the fuel cell is used as, for example, a home-use stationary power generator, the durability time is contemplated to have about 40000 hours. As such, the separator 4, 5 is required to have properties such as corrosion resistance sufficient to be durable against the long-time power generation.
Regarding the electrical conductivity, the contact resistance between the separator 4, 5 and the gas diffusion layer 2, 3 is desired to be as low as possible. A reason is that an increase of the contact resistance between the separator 4, 5 and the gas diffusion layer 2, 3 causes a reduction of the electric power generation efficiency of the proton-exchange membrane fuel cell. That is, as the contact resistance is lower, the electrical conductivity is higher.
Hitherto, a proton-exchange membrane fuel cell using graphite for a separator 4, 5 has been put into practical use. The separator 4, 5 formed of the graphite has advantages of having a relatively low contact resistance and not causing corrosion. On the other hand, however, the separator is easily damaged by shock. In addition, the separator has a disadvantage of increasing the processing costs for forming airflow channels 6 and hydrogen channels 7. These disadvantages of the separator 4, 5 formed of the graphite causes disturbance against popularization of proton-exchange membrane fuel cells.
As such, attempts have been made to apply metallic materials in lieu of the graphite as the material of the separator 4, 5. Particularly, various studies and approaches have been made to implement practical use of separators 4, 5 using stainless steels as materials.
For example, Japanese Unexamined Patent Application Publication No. 8-180883 discloses a technique using metal, such as stainless steel that readily forms a passive film. However, the formation of the passive film causes an increase in the contact resistance, leading to a decrease in the electric power generation efficiency. As such, problems have been pointed out regarding the matters in which the metallic materials have, for example, high contact resistances and low corrosion resistance in comparison with carbon materials such as graphite.
In addition, attempts have been made such that stainless steels not subjected to surface treatment are used as they are for separators. For example, Japanese Unexamined Patent Application Publications No. 2000-239806 and No. 2000-294256 each discloses a separator-use ferritic stainless steel formed such that Cu and Ni are positively added, impurity elements such as S, P, and N are then reduced, and C+N≦0.03 mass % and 10.5 mass %≦Cr+3×Mo≦43 mass % are satisfied. In addition, Japanese Unexamined Patent Application Publications No. 2000-265248 and No. 2000-294256 each discloses a separator-use ferritic stainless steel formed such that Cu and Ni are restricted to 0.2 mass % or less thereby to inhibit dissolution of metallic ions, impurity elements such as S, P, and N are then reduced, and C+N≦0.03 mass % and 10.5 mass %≦Cr+3×Mo≦43 mass % are satisfied.
Any of the inventions described above is based on the following idea. The composition of the stainless steel are controlled to the predetermined range to strengthen the passive film, whereby the stainless steel is not subjected to the surface treatment and is used as it is for reducing deterioration due to dissolved metallic ions in the catalytic activity of the electrode-carrying catalyst, thereby to restrain the increase in contact resistance with the electrode due to corrosion products. Thus, the stainless steel is not of the type that reduces the contact resistance itself of the stainless steel. Neither is a stainless steel of the type that enables securing the durability (i.e., output-voltage withstanding property) sufficient to withstand electric power generation over several tens of thousands of hours.
In addition, Japanese Unexamined Patent Application Publication No. 10-228914 discloses techniques enabling securing of high output in the manner that gold plating is applied to the surface of separators of, for example, a SUS 304 metallic material, thereby to reduce the contact resistance. However, with thin gold plating being performed, it is difficult prevent pinholes; and in contrast, with thick gold plating being performed, there remains a cost problem pending resolution.
Further, Japanese Unexamined Patent Application Publication No. 2000-277133 discloses techniques of obtaining separators improved in electrical conductivity in the manner that carbon powder is dispersed on a ferritic stainless steel base material. However, also in the case of using such powder, the surface treatment of the separator requires corresponding costs, still remaining cost problems pending resolution. In addition, a problem is pointed out in that in an event where the surface-treated separator is, for example, damaged during assembly, the corrosion resistance thereof decreases significantly.
Japanese Unexamined Patent Application Publication No. 2003-223904 discloses results of research and investigation of the influence of surface roughness of the surface of stainless steel. In this case, the stainless steel surface roughness is set as: Ra: 0.01 μm to 1 μm; and Ry: 0.01 to 20 μm. However, the contact resistance was found insufficient from the viewpoint of securing higher cell output.
In view of the above-described problems with the conventional techniques, it could be helpful to provide a stainless steel for a proton-exchange membrane fuel cell separator that has a high corrosion resistance and a low contact resistance (i.e., high electrical conductivity) and to provide a proton-exchange membrane fuel cell using the same.
More specifically, it could be helpful to provide a stainless steel for a proton-exchange membrane fuel cell separator and a proton-exchange membrane fuel cell using the same, wherein the proton-exchange membrane fuel cell separator is provided by specifying not only composition of stainless steel being as base material, but also composition of a passive film existing on the surface thereof, thereby to have a low contact resistance, a high electric power generation efficiency, and a high corrosion resistance of the stainless steel itself even without being subjected to a surface treatment.